1. Field of the Invention
The present invention relates to a coating method, and more particularly to a method of applying an organic solvent-based coating solution in which the coating solution is applied at high speed to a traveling flexible substrate such as a plastic film so as to form a thin layer thereon. Examples of the coating solution include a low-viscosity coating solution such as an undercoating solution for a magnetic recording medium, a back-layer coating solution and a surface treatment solution.
2. Background
In the process of manufacturing a magnetic recording medium or the like, it is known to apply a low-viscosity coating solution such as an undercoating solution for a magnetic coating solution, a back-layer coating solution, and a surface treatment solution. These low-viscosity coating solutions are organic solvent-based coating solutions. Conventional methods of applying a thin layer of such a coating solution to a flexible substrate, such as a plastic film, include utilizing various rod coaters, wire bar coaters, blade coaters, extrusion coaters, and the like.
For example, a conventional method and apparatus for applying a coating solution having a viscosity of 10 cp or less is disclosed in Japanese Patent Application (Laid-Open) No. 156066/1982. In particular, this application discloses a metering and coating method using a wire bar coater in which a wire is wound around a rod member for smoothing an excessively applied coating solution. Additionally, Japanese Patent Application (Laid-Open) No. 60750/1987 discloses a rod coater-type dust-removing method for removing dust together with a solvent while the solvent and the like are adhered to a rotating rod member. Further, Japanese Patent Application (Laid-Open) No. 131364/1991, discloses utilizing a doctor blade of a substrate-pressurizing type for scraping off a foreign substance on the substrate or in the coating solution and for leaving a predetermined amount of coating solution on the substrate. In these coating methods and apparatuses, since dust removal can be accomplished effectively by initially forming a relatively large amount of coating solution on the substrate, not only is the substrate not likely to be scratched, but the number of steaks in the remaining coating solution is relatively low. Hence, the above described methods and apparatuses are adequate for forming a thin-layered coating on a substrate.
However, in a case where the aforementioned coating solution is applied as an undercoating solution in the process of manufacturing, for instance, a magnetic recording medium, the following problems are conventionally encountered. Since an excess amount of the coating solution is applied to a dry substrate in an open state, not only is it difficult to increase the coating speed, but if the excess amount is increased to enhance the foreign-substance removing performance, a problem can occur in regard to its metering capability, and the amount of required coating solution increases, resulting in higher manufacturing costs. Furthermore, particularly in a case where the surface tension of the coating solution is high, there has been a drawback in that the spreadability of the coating significantly declines, thereby making it difficult to effect high-speed coating. With the recent advent of high-speed coating where a magnetic coating solution is applied to a substrate to form a magnetic recording medium, it has become necessary to increase the coating speed of an undercoating solution or the like, which conventionally has not presented much of a problem.
In coating such a coating solution described above, as a method of increasing the coating performance (wettability) of the coating solution, a method of providing corona discharge treatment or the like on the surface of the substrate has hitherto been widely used. However, it has been determined that this type of surface treatment of the substrate, for which large equipment and huge equipment cost are required, has not produced adequate results.
Furthermore, in applying the coating solution, in a case where the application of a thin layer of 10 cc/m.sup.2 or less is effected to reduce the running cost or increase the high-speed coating performance, it has been discovered that an unacceptable number of streaks are formed in the coating layer (i.e., the undercoating layer) due to foreign substances.
The present invention has been devised in view of the above-described problems, and its object is to provide a coating method which enables high-speed coating with a minimum of defects such as streaks even if the surface tension of the coating solution is high when an organic solvent-based coating solution of a low viscosity such as the one described above is applied to form a thin layer.